Synthesis and physicochemical properties of fac-[Re(CO)₃(κ²-N,N-dpkfah)Cl], dpkfah=di-2-pyridyl ketone 2-furoic acid hydrazone: the molecular structure of fac-[Re(CO)₃(κ²-N,N-dpkfah)Cl] · acetone

Abstract

Reaction between [Re(CO)₅Cl] and di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah) (1) in refluxing toluene gave fac-[Re(CO)₃(κ²-N,N-dpkfah)Cl] (2). Spectroscopic and electrochemical measurements disclosed sensitivity of 2 to its surroundings. ¹H-NMR measurements showed that the amide proton exchanged with solvent protons, and its chemical shift is solvent and temperature dependent, while the chemical shifts of aromatic protons are solvent and temperature independent. Electronic absorption spectra of 2 divulged two intra-ligand charge transfer transitions (ILCT) in protophilic solvents and a single ILCT transition in non-protophilic solvents. Optical measurements on protophilic solutions of 2 established an equilibrium between 2 and its conjugate base, fac-[Re(CO)₃(κ²-N,N-dpkfah-H)Cl]⁻ (3). Thermo-optical measurements confirmed that the interconversion between 2 and 3 and gave ΔG ^ø values of −26.48 and 22.99 kJ mol⁻¹, respectively, for the protonation of DMF and DMSO by 2. Optosensing measurements showed that [MCl₂] (M = Zn, Cd, or Hg) in concentrations as low as 1.00 × 10⁻⁷mol L⁻¹ can be detected and determined using protophilic solutions of 2. Electrochemical measurements showed 2 to be more stable in CH₃CN than DMF. Single-crystal X-ray structural analysis on fac-[Re(CO)₃(κ²-N,N-dpkfah)Cl] · acetone (4) obtained from an acetone solution of 2 confirmed the solvent–complex interaction and revealed two symmetry-independent molecules in the asymmetric unit. The extended structure of 4 disclosed parallel stacks connected via a network of classic and non-classic hydrogen bonds.

Keywords:

• Synthesis
• Rhenium
• Di-2-pyridyl ketone 2-furoic acid hydrazone
• X-ray crystallography
• Electrochemistry
• Spectroscopy

Acknowledgments

We acknowledge Ms Tony Johnson for NMR measurements and the University of the West Indies for funding of this research.

Notes

^†For the following inter-conversion:

where α = fac-[Re(CO)₃(κ²-N,N-dpkfah)Cl] and β ⁻ = fac-[Re(CO)₃(κ²-N,N-dpkfah-H)Cl]⁻, the total absorbance at wavelength λ equals , where c = [α] + [β] is the total concentration. Thus, and , so that the equilibrium constant K = [β]²/[a] at temperature T equals

(independent of λ and ϵ's are temperature independent because of the observed isosbestic points). The thermodynamic parameters characterizing the solvent protonation reaction (1) are then obtained according to